1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a semiconductor device having an electrode formed on a semiconductor substrate provided with a semiconductor integrated circuit and electrically connected to the semiconductor integrated circuit, an organic insulating film formed on the semiconductor substrate and having an opening through which the electrode is exposed, and a bump electrode formed on the electrode via a bump underlying metal film.
2. Background Art
In general, semiconductor devices have an external electrode for electric connection to outside. In recent years, from the standpoint of miniaturization and high performance of electronic appliances, high density mounting techniques, such as a flip chip system, have been put into practice, and these systems essentially require a bump (metallic protrusion) as an electrode for semiconductor devices. For a material of the external electrode of semiconductor devices, aluminium is usually employed. In this case, a bump electrode is formed on the aluminium electrode via a bump underlying metal film. One of methods of forming the bump underlying metal film includes an electroless plating method.
Referring now to FIG. 4, an instance of the steps of an electroless plating method will be described. A semiconductor device of FIG. 4 has such an arrangement that an aluminium electrode is formed on a silicon substrate formed with a semiconductor integrated circuit thereon, after which a polyimide film is formed as an insulating film, and an aluminium electrode is connected to outside via the opening formed at a given portion of the polyimide film.
FIG. 4(a) shows the formation, on a silicon substrate 9, of an aluminium film serving as an aluminium electrode 10 and a polyimide film 11 used as an insulating film. As shown in the figure, for connection of an external electrode to outside, the aluminium electrode 10 is exposed to through an opening 12 formed in position of the polyimide film 11, so that an oxide film 13 is formed on the surface of the aluminium electrode 10 by natural oxidation. In this condition, treatment is carried out so that the oxide film 13 is removed to activate the surface thereof. More particularly, wet etching is performed, for example, by immersion in an aqueous solution of phosphoric acid or sodium hydroxide. Further, a zincate treatment is effected wherein fine particles of zinc are deposited on the surface of the aluminium electrode 10 through substitution reaction.
Next, after cleaning with water, the semiconductor device is subjected to electroless plating by immersion in an electroless plating solution while heating to and keeping at 80xc2x0 C. by means of a heater, thereby providing a structure of FIG. 4(b). The electroless plating solution used is, for example, an electroless nickel (Ni) plating solution using sodium hypophosphite as a reducing agent. According to this step, phosphorus (P)-containing nickel precipitates from the fine particles of zinc on the aluminium surface, thereby forming an alloy film 14 of nickel and phosphorus (hereinafter referred to as Nixe2x80x94P film). At this stage, the Nixe2x80x94P film 14 should be formed in such a way that its thickness is smaller that that of the polyimide film 11 as is particularly depicted in FIG. 4(b).
After cleaning of the semiconductor device with water of room temperature after completion of the nickel plating, the semiconductor device is subjected to electroless gold plating by immersion of the device in an electroless gold (Au) plating solution heated to and kept at 60xc2x0 C. by means of a heater to provide a structure of FIG. 4(c). In this step, a gold film (hereinafter referred to as Au film) 15 is formed on the Nixe2x80x94P film 14 by substitution reaction with the gold ions in the electroless gold plating solution. The reason why the Au film 15 is formed on the Nixe2x80x94P film 14 is that when using the Nixe2x80x94P film 14 alone, connection becomes poor due to the existence of the oxide film formed on the surface and is enhanced by the provision of the Au film 15. Accordingly, the thickness of the Au film 15 is made very thin relative to the thickness of the Nixe2x80x94P film 14. After the formation of the Au film 15, the semiconductor device is cleaned with water to complete the electroless plating steps.
However, such an electroplating step as set out above has the following problem. As stated hereinabove, nickel plating is carried out by immersion in a nickel plating solution at a temperature of about 80xc2x0 C. and subsequent gold plating is carried out by immersion in a gold plating solution at a temperature of about 60xc2x0 C. On the other hand, when the polyimide film is compared with the Nixe2x80x94P film with respect to the coefficient of thermal expansion, the polyimide film has a coefficient of thermal expansion of 5xc3x9710xe2x88x925/xc2x0 C. and that of the Nixe2x80x94P film is at 13xc3x9710xe2x88x926/xc2x0 C., for one instance. In general, since the polyimide film has a coefficient of thermal expansion that is larger by one order of magnitude than that of the Nixe2x80x94P film, the polyimide film is larger than the Nixe2x80x94P film with respect to the shrinking rate ascribed to the temperature difference between the nickel plating solution and the gold plating solution. Accordingly, where the gold plating is carried out subsequently to the nickel plating, there is the possibility that the polyimide film separates from the Nixe2x80x94P film depending on the difference in shrinking rate between the polyimide film and the Nixe2x80x94P film caused by the temperature difference between the nickel plating solution and the gold plating solution, thereby causing an interstice or interstitial space to be created at the interface between the polyimide film and the Nixe2x80x94P film. When the gold plating is performed in such a state as to cause the interstitial space, the gold plating solution infiltrates from the space and arrives at the aluminium electrode, with the attendant problem that the acid component contained in the gold plating solution acts to corrode the aluminium electrode. Reference is now made to FIG. 5 to describe this phenomenon in more detail.
FIG. 5 is an electron photomicrograph by SEM (scanning electron microscope) of the semiconductor device obtained after the gold plating. Because the Au film is very thin, any Au film cannot be confirmed in the photograph. FIG. 5(a) shows the state where the polyimide film 11 and the Nixe2x80x94P film 14 are formed on the aluminium electrode 10. On the other hand, FIG. 5(b) is an enlarged view of FIG. 5(a) with respect to an interface S3 between the polyimide film 11 and the Nixe2x80x94P film 14. From FIG. 5(b), it will be seen that an interstitial space 16 is observed at the interface S3 between the polyimide film 11 and the Nixe2x80x94P film 14, and the aluminium electrode 10 beneath the space 16 is corroded. When the aluminium electrode 10 is corroded in this way, the electric characteristics and reliability of the resultant semiconductor device lower, with the problem that yield lowers.
FIG. 6 show an example of forming a solder bump on a bump underlying metal film formed according to a conventional electroless plating procedure. The end portion of the face where a solder bump 17 is in contact with the second metallic deposit film 15 corresponds to the vicinity of the interface between the polyimide film 11 and the Nixe2x80x94P film 10. Accordingly, stress is concentrated on this portion, with the attendant problem that such concentration leads to the breakage of the electrode.
The invention has been made in order to overcome these problems. More particularly, the invention provides a semiconductor device wherein the corrosion of the electrode caused through the interstitial space created at the interface between an organic insulating film and a metallic deposit film can be prevented, thereby providing excellent electric characteristics and reliability.
The present invention relates to a semiconductor device comprising: an electrode formed on a semiconductor substrate provided with a semiconductor integrated circuit and electrically connected to the semiconductor integrated circuit; an organic insulating film formed on the semiconductor substrate and having an opening through which the electrode is exposed: and a bump electrode formed on the electrode through a bump underlying metal film. According to one aspect of the present invention, in the semiconductor device; the bump underlying metal film includes a first metallic deposit film formed by a first electroless plating method and a second metallic deposit film formed on the first metallic deposit film by a second electroless plating method using a plating solution different in type from that of the first electroless plating method. The first metallic deposit film has a thickness larger than said organic insulating film. The first metallic deposit film is so formed that a peripheral portion thereof is superposed on the organic insulating film.